Conventionally, there are various pressing force sensors using a flat membrane-shaped piezoelectric element. Patent Document 1 describes a pressing force sensor having a flat membrane-shaped piezoelectric element formed on a main surface of a flexible substrate. Referring to the pressing force sensor described in the Patent Document 1, a piezoelectric crystalline thin film is formed on a whole surface of a flexible substrate having a square main surface. The flexible substrate is supported by a support so as to be placed at predetermined intervals with respect to a ground substrate disposed on a back side. The support is disposed to take a square shape in a planar view of the flexible substrate. Consequently, a central area on the main surface of the flexible substrate tends to be dented by pressing force applied in an orthogonal direction to the main surface and the pressing force is detected from voltage generated by the dent.
The piezoelectric crystalline thin film described in the Patent Document 1 is formed of composite oxide containing metal as a main component. For this reason, the piezoelectric crystalline thin film is fragile to external force or predetermined detection sensitivity cannot be obtained depending on a formation state thereof in some cases.
For a piezoelectric element which overcomes defects caused by the piezoelectric crystalline thin film due to the composite oxide containing metal as a main component, it is supposed to use a piezoelectric sheet containing polylactic acid as a main component. It is known that a flat membrane-shaped piezoelectric element using the piezoelectric sheet containing the polylactic acid as the main component is excellent in flexibility and sufficient voltage can be obtained for detecting pressing force. Moreover, the piezoelectric sheet containing the polylactic acid as the main component is extended wholly by the pressing force. Referring to the piezoelectric sheet containing the lactic acid as the main component, accordingly, it is possible to detect, as distortion of the piezoelectric film, the extension as well as the dent of the piezoelectric sheet. Therefore, it is possible to detect the pressing force more efficiently as compared with the piezoelectric sheet disclosed in the Patent Document 1.
The piezoelectric sheet containing the polylactic acid has a piezoelectric constant d14. The flat membrane-shaped piezoelectric element using the piezoelectric sheet obtains detection voltage by utilizing piezoelectricity caused by the piezoelectric constant d14.
Patent Document 1: Japanese Unexamined Patent Publication No. 2000-275114